Leo Szilard was sitting in the lobby of his hotel in London reading the Times. It was Tuesday, September 12, 1933. In that issue, the newspaper featured an in-depth report on a scientific conference where the renowned physicist Ernest Rutherford had lectured on the theories others had advanced of gathering and using the energy which was supposedly stored in atoms. Journalists gave a detailed account of Rutherford’s lecture, one that annoyed Szilard immensely. Rutherford claimed that “anyone who expects to find a source of power by transforming atoms is talking moonshine”, and most of the scientists of the time believed him. But Szilard thought otherwise.

Rutherford’s statements on the impossibility of nuclear energy made Szilard so angry that he decided to dedicate himself to solving this problem and prove the famous physicist wrong. He left the hotel and took a walk through the city to clear his mind. Before he had strolled very far down one of the nearby streets an idea flashed before him. He had thought of a way to release the energy of an atom. “I stopped for a red light at the intersection of Southampton Row [at Russell Square],” he later recalled. In a few moments cars started to slow down and stop and pedestrians were able to cross the road. It was then that Szilard realized how it would be possible to release the enormous amounts of energy stored in every atom. He would have to create a chain reaction using neutrons.

“Eureka” at the pedestrian crossing

“As the light changed to green and I crossed the street, it suddenly occurred to me that if we could find an element which is split by neutrons and emit two neutrons when it absorbed one neutron, such an element, if assembled in sufficiently large mass, could sustain a nuclear chain reaction. I didn’t see at the moment just how one would go about finding such an element, or what experiments would be needed, but the idea never left me. In certain circumstances it might be possible to set up a nuclear chain reaction, liberate energy on an industrial scale, and construct atomic bombs.”

During the winter, Szilard put his ideas down on paper. The concept he was dealing with was completely new: the existence of the neutron particle had only been proven a year earlier. The crucial discovery was that neutrons, unlike protons with which they constitute the nucleus of an atom, are electrically neutral and are not influenced by the electrical repulsive force, so they could penetrate the nucleus from outside and reshape it in such a way that it would become unstable and split it into two parts.

Naturally, finding an element which would emit energy and free neutrons after being split up was essential. According to Szilard, this method of inducing what he called nuclear fission would result in the release of energy, stored up to that point in the atomic nucleus. When this nucleus is split apart by the neutrons, two new nuclei are created: because the combined energy of these new particles would, he argued, be smaller than that of the original nucleus, the excess energy would necessarily be released in the form of heat. If this moment of fission released further neutrons, which then collided in the same way with neighboring nuclei, a chain reaction would take place, releasing unimaginable amounts of energy.

The most important challenge in achieving this was to find an element which would, during fission, release energy as well as free neutrons. At first Szilard surmised that the appropriate elements were beryllium and iridium, but this supposition turned out to be wrong. Only later did it become evident that the most suitable element for releasing nuclear energy was uranium.

A writer’s prediction of the future proves more accurate than a scientist’s

To finance his research he first turned to a factory-owner Hugo Hirst. It is interesting that he did not enclose scientific articles with his letter of request, but a passage from the novel by H. G. Wells, The World Set Free. In the novel, a scientist called Holsten invents a way to release the energy stored in atoms. “Of course, all this is moonshine,” Szilard wrote in his letter to the factory-owner, intentionally citing Rutherford’s lecture, “but I have reason to believe that when it comes to seeing what industry might do with the latest discoveries in physics, writers of fiction may well be offering more accurate predictions than scientists.” It was none other than H. G. Wells who invented the term “atom bomb”.

Soon after this, Szilard faced an important decision. Should he reveal his findings on nuclear energy to the public by publishing them and risk his idea going into the wrong hands, or should he keep it a carefully protected secret? Holsten, the scientist from H. G. Wells’s novel, decided that he was no more than an insignificant instrument in the grand machinery of progress and change. Even if he burned all his papers, someone else would come to the same conclusions in a few years. But Szilard decided otherwise. Instead of publishing his idea in a scientific journal, he decided to keep it secret.

First he thoroughly described his discovery of how the energy of atoms might be tapped and exploited. Then he patented his idea of critical mass and the chain reaction produced by means of neutrons. In 1935 he transferred the rights to the patent to the British Army on condition that it remained an absolute secret. During the following years leading up to the Second World War, all those with clearance to read Szilard’s work did their utmost to prevent it reaching Hitler’s scientists. Meanwhile in 1939 Szilard confirmed his theory in the laboratory. In February that year, an experiment was carried out at Columbia University, New York which later led to the first controlled chain reaction, and opened the era of nuclear energy. Szilard built the nuclear reactor in Manhattan with Enrico Fermi, an expert on bombarding atomic nuclei with neutrons. Several years earlier in Rome, Fermi had already discovered that decelerated neutrons fuse better with atomic nuclei than faster neutrons, so it was important to find a substance that could slow neutrons down. One suitable option was heavy water (water highly enriched in the hydrogen isotope deuterium), but this was difficult to obtain in sufficient quantities. To perform the experiments a more accessible alternative had to be found.

How to beat the Nazis?

As Szilard feared, German scientists were also thinking of ways to build a nuclear reactor. However, they had the same difficulties obtaining sufficient heavy water, forcing them also to search for another solution. Both teams tested graphite, but fortunately the Germans gave up on this possibility after concluding, too hastily, that graphite absorbed too many neutrons. Szilard, on the other hand, realized that the problem lay not with the graphite itself, but the quantities of boron added to it in the manufacturing process. Pure graphite in fact proved highly suitable as a “brake” for slowing down neutrons in a nuclear reactor. When this became apparent, Szilard was much more aware of the gravity of the moment than Fermi. In the summer of 1939, Fermi left for the University of Michigan to study cosmic radiation, while Szilard decided to turn to his old colleague Albert Einstein who, at the time, lived on Long Island, a hundred or so kilometers outside New York. Szilard presented his idea, which was actually based on Einstein’s famous and revolutionary equation E=mc2. Einstein responded by saying that he had never thought of anything like this when he thought of how his proof relating mass to energy might be applied.

On his second visit to Einstein, Szilard brought with him a letter he had written to President Roosevelt warning him of the danger that Hitler’s Germany might also be trying to create an atomic weapon from nuclear fission, and suggesting that the U.S should seriously consider developing the technology itself. It is reported that Einstein read and signed the letter, which later turned out to be extremely important, wearing only his dressing gown. The letter began with the words “Some recent work by E. Fermi and L. Szilard…” and was dated August 2nd 1939.

Of course, Szilard did not simply send the letter to the President by mail as it might have come into the wrong hands, but instead asked an acquaintance in the administration, one of the President’s economic advisers, to take it directly to the White House. When Roosevelt finally read the letter he realized that something absolutely must be done “to see that the Nazis don’t blow us up”. He founded the Uranium Committee, of which Szilard was made a member, and which set in motion the research that ultimately led to the creation of the atom bomb. It also brought about the nuclear reactors which we use to produce great amounts of electrical energy today, and which many environmentalists still view as one of the most acceptable alternative energy sources in the ongoing fight against global climate change.